1. Field of the Invention
The present invention relates in general to a short-pulse amplification technique which reduces or avoids nonlinear effects, thus allowing higher pulse energies without pulse distortion. A pulse to be amplified is divided into a selected number of smaller magnitude pulses that are otherwise identical in shape to the original pulse. The pulses are amplified and then recombined coherently to produce a final output pulse that is an amplified version of the original pulse.
2. Description of the Background Art
Ultrashort light pulses on the order of picoseconds or shorter are now finding application in a wide range of science and technology. Many applications require high-energy pulses, which are obtained by amplifying low-energy pulses generated by a laser. Amplification of the pulses with high fidelity is crucial, but nonlinear phase shifts accumulated by an intense pulse generally distort its spectral and temporal profiles. Dispersion management, exemplified by chirped-pulse amplification, has proved to be an effective way to control nonlinearity. However, the limits of chirped-pulse amplification are reached by many sources of picosecond and femtosecond pulses.
If an optical pulse accumulates a nonlinear phase shift
            Φ      NL        ⁡          (              t        ,        z            )        =            ω      c        ⁢          ∫                        n          2                ⁢                  I          ⁡                      (                          t              ,              z                        )                          ⁢                  ⅆ          z                    (where I is the intensity and n2 is the nonlinear refractive index of the medium) that is greater than ˜1, its spectral, temporal, and/or spatial profiles are likely to be distorted. In chirped-pulse amplification (CPA), a pulse is stretched temporally by a dispersive delay line. The stretched (thus frequency-chirped) pulse is amplified, and then the pulse is dechirped to its initial duration in another dispersive delay line. This technique reduces the intensity when the pulse is propagating through the (solid) amplifying medium, and allows the intensity to be a maximum when the pulse is propagating linearly. Self-similar amplification also controls nonlinearity through dispersion.
Short-pulse amplifiers based on CPA have been responsible for a major fraction of the ultrafast science performed to date. There is ongoing interest in the generation of ultrashort pulses with ever-higher energies, for applications such as the generation of attosecond pulses and ultrafast x-rays. Many existing CPA systems are operated close to the limit ΦNL˜1, owing to limitations on the stretching ratio. In practice, it is difficult to stretch and compress a pulse by more than a factor of 104 with high fidelity. As a separate issue, practical devices that provide enough dispersion to stretch and compress high-energy pulses longer than a few picoseconds do not exist.